In recent years, an organic material that exhibits high heat resistance (e.g., polyimide resin) has been widely applied as a protective film material for a semiconductor integrated circuit (LSI) (see Patent Documents 1 and 2, for example).
A protective film (cured film) that utilizes a polyimide resin is obtained by applying a polyimide precursor or a resin composition that includes a polyimide precursor to a substrate, followed by drying to form a resin film, and curing the resin film by heating.
It is has been desired to reduce the dielectric constant of an interlayer insulating film (low-k layer) along with miniaturization of a semiconductor integrated circuit. The dielectric constant may be reduced by utilizing an interlayer insulating film having a porous structure. However, this method has a problem in that a decrease in mechanical strength occurs. A protective film may be provided on an interlayer insulating film that exhibits low mechanical strength in order to protect the interlayer insulating film.
An increase in thickness (e.g., 5 μm or more) and an increase in modulus of elasticity (e.g., 4 GPa or more) have been desired for such a protective film in order to prevent a situation in which stress is concentrated on the interlayer insulating film in an area in which a bump (protruding external electrode) is formed, and the interlayer insulating film breaks. However, when the thickness and the modulus of elasticity of the protective film are increased, the stress of the protective film may increase, and the semiconductor wafer may be warped to a large extent, whereby a problem may occur when transferring or securing the wafer. Therefore, development of a polyimide resin that exhibits low stress has been desired.
The stress of the polyimide resin may be reduced by causing the molecular chain of the polyimide to have a rigid skeleton in order to bring the coefficient of thermal expansion of the polyimide closer to the coefficient of thermal expansion of a silicon wafer (see Patent Document 3, for example), or introducing a flexible structure such as a siloxane structure into the polyimide to reduce the modulus of elasticity of the polyimide (see Patent Document 4, for example), for example.
When the polyimide resin that is used to form the protective film exhibits photosensitivity, it is possible to easily form a patterned resin film. A patterned cured film can be easily formed by curing such a patterned resin film by heating.
A polyimide resin that exhibits photosensitivity may be obtained by providing the polyimide with photosensitivity. It is possible to provide the polyimide with photosensitivity by introducing a methacryloyl group into the polyimide precursor through an ester bond or an ionic bond, utilizing a soluble polyimide that includes a photopolymerizable olefin, or utilizing a self-sensitizing polyimide that has a benzophenone skeleton, and includes an alkyl group at the ortho-position of the aromatic ring to which a nitrogen atom is bonded (see Patent Document 5, for example), for example. The method that introduces a methacryloyl group into the polyimide precursor through an ester bond has advantages in that it is possible to arbitrarily select a monomer when synthesizing the polyimide precursor, and excellent temporal stability is achieved since a methacryloyl group is introduced through a chemical bond.
However, when a large amount of aromatic ring units are introduced in the polyimide resin that exhibits low stress in order to cause the molecular chain to have a rigid skeleton, a large amount of conjugate aromatic ring units are included in the molecular chain, and even an polyamic acid (polyimide precursor) (i.e., a precursor of polyimide resin) has absorption in the ultraviolet region. Therefore, the transmittance of an i-line (wavelength: 365 nm) that is widely used for an exposure step for forming a patterned resin film may decrease, and the sensitivity and the resolution tend to decrease. When the thickness of the protective film is increased, the i-line transmittance may further decrease, and it may be difficult to form a patterned resin film. A decrease in heat resistance may occur when a flexible structure such as a siloxane structure is introduced.